I'm a charter member of the "I Hate Dook Goo Kau Bai and Think He's Overrated Society," but even I wouldn't call him "2nd or 3rd rate." I have no problem believing or accepting that he was likely a terrific swordsman, but it does rankle me when people assume that he can kill all the Greats by just glancing sideways at them (which some of his ardent fans are wont to do).
I think Dook Goo Kau Bai was good enough to defeat any of the CONDOR HEROES TRILOGY Greats one-on-one after a protracted struggle, but it won't be a quick, one-sided fight, and two or more Greats would probably overwhelm him.